The present invention relates generally to sealing rings and, more particularly, to an improved split sealing ring to prevent extrusion damage to a packing or the like.
It is generally well known that most hydraulic or pneumatic systems employ elastomeric and/or plastic seals in order to prevent or control the flow or passage of fluid through the clearance between two closely fitting surfaces, generally cylindrical surfaces, such as a piston and cylinder, piston rod and rod guide or a rotating shaft and its surrounding housing. The closely fitting surfaces may be dynamic or movable with respect to each other, either in the axial or circumferential direction, or may be static or stationary with respect to each other.
In general, the seal is installed in a machined groove extending into one of the two surfaces to be sealed. Typically, the cross section of the seal is at least slightly larger than the cross section of the seal-receiving groove so that when the two surfaces to be sealed are brought together to form a gland, a portion of the cross section of the seal is squeezed, thereby absorbing the tolerance backup between the surfaces. When the seal is an elastomeric material, the elastomer is generally a highly viscous, incompressible fluid with a high surface tension, thereby giving the seal a "memory" or the tendency to return to its original shape. In low pressure applications, where the fluid being sealed exerts little or no pressure or force on the seal, as the seal is squeezed or deformed into the gland, the seal exerts a return force against the mating surface and the groove. In this manner, the seal firmly contacts both the mating surface and the groove to create a barrier for blocking the passage of fluid between the surfaces.
For applications in which higher pressures are exerted on the seal by the fluid, the sealing force of the squeezed seal is augmented by the system fluid pressure as it is transmitted through the elastomeric seal. FIG. 7 illustrates a typical high pressure seal assembly which employs an elastomeric seal element 60 which is shown as being T-shaped in cross-section. The seal element 60 is installed within a groove 62 cut within surface 64 for sealing the clearance 66 between curved surfaces 64 and 68. The pressure is typically transmitted through the portion of the seal element 60 engaging the groove (known as the "flange") which activates one or more generally rigid back-up rings or anti-extrusion rings 70. The back-up rings 70 are forced out of the groove and into the clearance 66 between the two curved surfaces 64 and 68. In sealing systems of this type, the primary function of the back-up rings 70 is to prevent the softer elastomeric seal element 60 or packing from being damaged as a result of its being forced or extruded into the clearance 66.
PTFE is a typical material used for the manufacture of such back-up rings since the PTFE provides adequate extrusion resistance for fluid pressure up to approximately 3000 p.s.i. Alternatively, such back-up rings could be made of another high strength generally rigid material such as Nylon, Peek, filled PTFE or even metal, depending upon the fluid pressures involved. In order to facilitate the installation of a rigid back-up ring of this type into a typical seal gland on or around a piston or in a rod housing, the back-up ring is split or cut completely therethrough at one place on its circumference. Of course, once the ring is cut, it no longer provides a perfect, endless circumferential plane of protection for the seal or packing element of the seal assembly. As a result, the back-up ring tends to separate at the split, particularly in high pressure applications, permitting the softer seal element or packing element to extrude into the resulting gap in the back-up ring, causing deterioration of the seal element.
Numerous methods have previously been attempted and employed in order to prevent or minimize the gap in the back-up ring and to thereby prevent such extrusion of the seal element. One such method involves placing the cut in the back-up ring at an angle with respect to the axial surfaces of the ring with the cut extending from one axial surface to the other axial surface. This type of cut, known as a "scarf" cut is illustrated in FIGS. 1A and 1B. While the scarf cut is effective in minimizing the gap in the back-up ring with respect to forces or extrusion in the axial direction (i.e., into or out of the paper when viewing FIG. 1A), this cut provides virtually no protection for forces or extrusion extending in the radial direction (i.e., into or out of the paper when viewing FIG. 1B).
A second method of maintaining the gap in the back-up ring to a minimum involves making the cut along an angle with respect to a tangent on a circumferential surface of the ring with the cut extending from the outer to the inner circumferential surfaces of the back-up ring. This type of prior art cut, known as a "skive" cut is illustrated in FIGS. 2A and 2B. The skive cut is effective in maintaining minimum gap size and minimum extrusion with respect to forces or extrusion extending in the radial direction (i.e., into or out of the paper when viewing FIG. 2B), but affords little or no protection with respect to forces or extrusion extending in the axial direction (i.e., into or out of the paper when viewing FIG. 2A).
While seal assemblies having a back-up ring with a scarf cut are effective in some applications and seal assemblies with a back-up ring having a skive cut are effective in other applications, there are applications in which the back-up ring experiences both axial and radial forces. In such applications, neither of these back-up rings is particularly effective. In addition, it is desirable to have a seal assembly of a single design which is effective for a variety of different sealing applications in which the back-up ring can experience axial or radial forces, or both, and still properly function. The present invention overcomes the disadvantages of seal assemblies having a back-up ring with either a scarf or a skive cut by providing a cut, referred to as a "bias" cut which, when properly configured, provides efficient extrusion protection with respect to forces in both the axial and radial directions. The present invention relates to a split sealing ring in which the cut is a combination of both the scarf and skive cuts to provide both an axial and a radial bearing plane at the split, thereby affording the advantages of both of these prior cuts in a single sealing ring.